22/03/1999 A.Blas 1

Hollow bunchesHollow bunchesA. Blas, S. Hancock, S. Koscielniak, M. Lindroos, F. Pedersen, A. Blas, S. Hancock, S. Koscielniak, M. Lindroos, F. Pedersen,

H. SchonauerH. Schonauer

Why: Why: to improve space charge related to improve space charge related problems.problems.

How: How: Increase the IIncrease the Imean mean / I/ Ipeakpeak= BF value by = BF value by creating a hollow distribution in the creating a hollow distribution in the longitudinal phase spacelongitudinal phase space

22/03/1999 A.Blas 2

Hollow bunches (history)Hollow bunches (history)

F. Pedersen (PSB 1978): F. Pedersen (PSB 1978): h = 5 bucket deposit in a 50 MeV coasting h = 5 bucket deposit in a 50 MeV coasting beam and acceleration with h = 5 and 7.10beam and acceleration with h = 5 and 7.101010 p (no fast FB!) . p (no fast FB!) .

K. Schindl (PSB 1978): K. Schindl (PSB 1978): Use of dual harmonic debuncher in the Use of dual harmonic debuncher in the Linac transfer lineLinac transfer line

ResultsResults: : loss of hollowness when closing any loop. Abandoned because loss of hollowness when closing any loop. Abandoned because of the success of dual harmonic (h5 + h10) operation.of the success of dual harmonic (h5 + h10) operation.

R. Garoby, S. Hancock (PS 1992): R. Garoby, S. Hancock (PS 1992): Phase shaking at ~ 0.94 fPhase shaking at ~ 0.94 fSS and and homogenization with 200 MHz cavity. Very successful from 1GeV homogenization with 200 MHz cavity. Very successful from 1GeV to 26 GeV trough transition.to 26 GeV trough transition.

S. Hancock (PSB 1997): S. Hancock (PSB 1997): same method as above, but h=16 same method as above, but h=16 frequency control too poor at that time to get proper results.frequency control too poor at that time to get proper results.

22/03/1999 A.Blas 3

Pre-requisitePre-requisite

BTFM: BTFM: longitudinal beam transfer function longitudinal beam transfer function measurement (H. Schonauer, M. Sjöström)measurement (H. Schonauer, M. Sjöström)

Energy width measurement for 1.1 turns injected (1E12 pp).Left fig: BTFM (550 keV FW) right fig: Linac spectrometer (405 keV FW).

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Bucket deposition Bucket deposition T [eV]

t [s]

C16 Buckets

Coasting beam

T [eV]

t [s]

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Bucket depositionBucket deposition

Computer simulation of longitudinal phase space after deposition of V16=1kV empty buckets; T=5ms

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Bucket depositionBucket deposition

Energy spectrum (from BTFM) after deposition of empty buckets with V15

=4kV, 8ms frequencysweep

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Bucket deposition (rf set-up)Bucket deposition (rf set-up)

An “empty” bucket is An “empty” bucket is brought into the brought into the injected beam before injected beam before capturecapture C16 cavity for “hole-

generation” C02 cavity for

acceleration (h=1) C04 cavity for bunch

shaping

Time (s)

Voltage (kV)

310 315 320 325 330 335

Time (s)

Frequency (kHz)

310 315 320 325 330

Frequency of the “synchronous” particle

C16 frequency

Sweep time

40 kHz

4

8

C16

C02

C04

injection

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1. Single Harmonic1. Single Harmonic1.2 1D Line density1.2 1D Line density

1D Line density

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1. Single Harmonic 1. Single Harmonic

C02=8 kVC02=8 kV C16=6 kVC16=6 kV

Red: 8 ms sweep Black: 1 ms sweep

Energy = 50 MeVEnergy = 50 MeV 3 turns injected (?)3 turns injected (?)

Red Black

Emittance(90%) eVs

1.09 1.15

Emittance(rms) eVs

1.80 1.39

2D density profiles

Synchrotron frequency

2D density profiles

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2. Dual Harmonic2. Dual Harmonic2.4 Line density2.4 Line density

2D density profile 1D line density

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2. Dual harmonic2. Dual harmonic2.1 Amplitude distribution2.1 Amplitude distribution

C02=8 kVC02=8 kV C04=4 kVC04=4 kV C16C16

red: 0 kV and no sweep green: 3 kV and 0.5 ms sweep blue: 3 kV and 8 ms sweep lilac: 6 kV and 8 ms sweep

Energy: 50 MeVEnergy: 50 MeV 3 turns injected3 turns injected

2D Density Profile

Synchrotron frequency

fS

p/eV.s

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2. Dual Harmonic2. Dual Harmonic2.2 Emittance2.2 Emittance

Red Green blue lilac

Emittance(90%) eVs

0.79 0.92 0.94 0.87

Emittance(rms) eVs

0.82 1.00 1.34 1.56

2D density profiles

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2. Dual harmonic2. Dual harmonic2.3 Phase space images2.3 Phase space images

p/eV.s

fS

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3. Dual harmonic instability3. Dual harmonic instability3.1 Time development3.1 Time development

C02=8 kVC02=8 kV C04=4 kVC04=4 kV Energy: 212 MeV at Energy: 212 MeV at

instability (green)instability (green)

2D density profiles

Synchrotron frequency

Black Green Red

Emittance(90%) eVs

1.15 1.19 1.18

Emittance(rms) eVs

1.17 1.20 1.11

Before instability

During instability

After instability

129 MeV 212 MeV 655 MeV

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3. Dual harmonic instability3. Dual harmonic instability3.2 “Phase space” before, during and after3.2 “Phase space” before, during and after

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3. Dual harmonic instability3. Dual harmonic instability3.3 Instability frequency3.3 Instability frequency

Basic repetitions rate: 4184.3 HzBasic repetitions rate: 4184.3 Hz

If octupolar: f = 4184.3 / 4 If octupolar: f = 4184.3 / 4 1046 Hz 1046 Hz

fsynch.=1046 Hz

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ConclusionConclusion

Bunching factor is increased Bunching factor is increased 0.28 to 0.38 in the single harmonic case0.28 to 0.38 in the single harmonic case

0.45 to 0.55 in the dual harmonic case0.45 to 0.55 in the dual harmonic case

(values at 100 MeV)(values at 100 MeV)

Single harmonic acceleration was Single harmonic acceleration was successfulsuccessful...but loss of hollowness after 390 ms...but loss of hollowness after 390 ms

Dual harmonic operation unstableDual harmonic operation unstable

...from 50 ms to 250 ms...from 50 ms to 250 ms